The present invention relates to a magnetic brush development apparatus for use with an electrophotographic copying machine or electrostatic recording apparatus.
The magnetic brush development apparatus is an apparatus for attracting powder-like developer containing magnetic powder onto a development roller to form a magnetic brush which simulates the fibers of a brush thereon, and for bringing the magnetic brush into contact with a latent electrostatic image formed on the surface of a photoconductor or of a recording member, so that the latent electrostatic image is developed. Usually, the development roller comprises a rotatable non-magnetic sleeve with an internally disposed stationary magnet having a plurality of magnetic poles on its peripheral surface.
It is particularly important that the development roller attracts the developer to its surface and brings the magnetic brush formed on its surface into contact with the surface of a photoconductor only during development, and moves the developer away from the surface of the photoconductor when development is not performed. Particularly in the case of multi-color development in color copying apparatus, when a specific latent electrostatic image is developed by a specific color developer, other color developers must be precluded from being brought into contact with the electrostatic image. Otherwise, the developed image will be disturbed and smeared with other color developers.
In order to keep the developer on the development roller off a development surface when development is not performed, a variety of methods have been proposed. In one method, the development roller is designed so as to be moved away from the surface of a photoconductor when development is not performed. A shortcoming of this method is that a complicated mechanism is required in order to keep the gap between the development roller and the surface of the photoconductor constant and its operation is unstable and its cost is too high to be used. Furthermore, movable members for use in the mechanism are generally large in size and therefore require a great space for moving the development roller away from the photoconductor in a copying machine, so that the use of such mechanism is disadvantageous for reducing the size of the copying machine. Furthermore, this mechanism requires a number of members be disposed around the photoconductor, so that there is a risk that the surface of the photoconductor can be scratched or damaged by the members when the photoconductor is exchanged with a new photoconductor.
In another conventional method, a blade for scraping developer from the surface of a development roller is disposed in contact with the surface of the development roller, and, when development is not performed, the development roller is rotated in a reverse direction so that the developer on the development roller is removed by the blade. In a further conventional method, a blade for regulating the amount of developer on a development roller is disposed in a detachable manner from the surface of the development roller, and, when development is not performed, the blade is brought into contact with the surface of the development roller to scrape developer from the development roller. In the former method, a complicated and expensive drive source is required and a great torque is applied to the development roller, so that a copying machine using this method is great in size, if the necessary drive and control mechanisms are included. In the latter method, the necessary apparatus is rather simple in the mechanism and economical. However, there is a limitation to the attachment position of the blade with respect to the development roller, which makes the necessary mechanism complicated and reduces operational stability in practice. In this method, it is proposed to apply a bias voltage to a development roller in order to improve development efficiency. In this case, however, it is extremely difficult to insulate the blade electrically and a more complicated mechanism is required in order to overcome this problem. Further, since the blade is brought directly into contact with the surface of the development roller, the development roller may be smeared, damaged or charged triboelectrically, having adverse effects on development. The adverse effects cannot be ignored. Therefore, in order to overcome these problems, there are not many choices in the suitable materials for the development roller and the blade and the suitable methods for surface treatment of the development roller and the blade. As a result, this method is concluded to be an expensive method. The above-mentioned shortcomings have adverse effects on the quality of developed images and the overall life and reliability of a copying machine.